1. Field of the Invention
The present invention is directed to reflective liquid crystal (LC) lightvalves comprising a twisted nematic LC layer whose molecules are aligned with pixel edges at the mirror backplane, resulting in improved contrast and reduced visibility of post spacers in black state.
2. Prior Art
Reflection lightvalves are becoming widely popular for use in projection displays. Such lightvalves can be reduced in size with relatively little loss in pixel aperture, allowing a corresponding shrinkage in size and cost of the projection system. Reflective lightvalves based on twisted nematic liquid crystal (TNLC) layers, such as the 45xc2x0 C. twist or 54xc2x0 C. twist modes, make use of well developed LC technology, and with relatively modest driving voltages can provide a reasonably satisfactory optical response when reproducing black, white or intermediate grayshaded image regions; however, the image appearance in black state exhibits certain imperfections which will now be described. Conventionally, the above described systems are illuminated with light that is polarized in a direction that is rectilinear with the x,y axes of lightvalve pixels, i.e. the incident e-field is parallel or perpendicular to the pixel edges. The e-field at the mirror backplane is then rotated by the twist angle relative to this incident direction, e.g., the polarization at the backplane will be rotated by 45xc2x0 or 54xc2x0 relative to the pixel edges.
The topography of the backplane typically comprises vertical and horizontal mirror electrode edges, the pixel mirrors being laid out in a row, column fashion to provide the pixels of the projected image. When a conductor like these pixel electrodes is illuminated by an electromagnetic (EandM) field, currents are set up along its boundaries, giving rise to scattered radiation. When the edges are straight the polarization eigenstates of the scattered radiation are approximately rectilinear with these edges. If the input polarization is not one of these eigenstates, e.g., if it is in a rotated orientation, the scattered light will tend to be depolarized by the edges. In order to avoid depolarization from scattering by pixel electrode topography, it would be highly desirable to have the polarization at the backplane be horizontal or vertical, instead of oriented at e.g. 45xc2x0 or 54xc2x0. Such depolarization adds unwanted light to the black state image, and removes useful light from the white state image. The pixels that produce this scattered light have the same periodicity as the diffraction orders which carry the image information, making it impossible to remove the depolarized light by spatial filtering, as might otherwise be possible if a laser illumination source were used.
The most common manufacturing process for establishing the orientation of the LC molecules at the backplane is through rubbing of an alignment layer. This rubbing process creates artifacts when the lightvalve cell gap is maintained by spacer posts placed in the boundaries between mirror pixels. The principle advantage of spacer post technology is that it provides very accurate control of cell gap; however a disadvantage is that spacer posts perturb the alignment of nearby LC. Incident light whose polarization is altered by that portion of the disturbed LC which is immediately adjacent to the posts will largely be absorbed by the low reflectivity layer that separates the pixel mirrors; thus, in regions very close to the posts, the disturbed LC has little effect on the displayed image. Unfortunately, the region of disturbed LC may be considerably extended (xcx9c10 xcexcm) in the direction of alignment layer rubbing. In the known reflective TN lightvalves this rubbing direction is at an angle such as 45xc2x0 to the dark inter-pixel boundaries, creating visible LC disturbance in the regions over the mirrors.
The present invention is directed to a liquid crystal (LC) structure wherein the backplane is rubbed in a direction rectilinear with pixel edges. The LC layer is given the same twist rotation and birefringence as in the conventional TN lightvalve. Polarization control is maintained by illuminating the lightvalve with light whose polarization is rotated by the twist angle relative to the x,y, pixel axes, and by collection of the orthogonally polarized component of the reflected light. The lightvalve top glass is thus rubbed in a direction which is rotated by the twist angle from the horizontal or vertical direction at which the backplane is rubbed.
Moreover, in the present invention, several methods are disclosed to provide illumination and collection in the desired polarization directions: First, light may be introduced through a polarizing beamsplitter (PBS) or 1-PBS, e.g., PBS+plumbicon prisms, optics which are rotated so as to place the P plane of the PBS hypotenuse coating into an orientation where its intersection with the lightvalve plane is rectilinear with the desired illumination polarization.
Second, a tilted DBEF beamsplitter may be used, with pass axis rotated within the substrate plane to the angle of the desired collection polarization, or to the perpendicular angle. DBEF beamsplitter is a product of 3M Corporation.
Third, an existing optical system in which light is obliquely incident through one polarizer onto the lightvalve, and in which light is collected through an offset orthogonal polarizer, may be utilized but with rotators such as halfwave rotators placed between the color dichroics and the linear polarizers.
Fourth, a PBS or 1-PBS set of optics in the conventional orientation may be used, with a precision achromatic halfwave retarder placed between the optics and the lightvalve.
Fifth, a PBS or 1-PBS set of optics may be used in the conventional orientation, together with a twist layer to rotate the input polarization.
Sixth, a PBS or 1-PBS set of optics may be used in the conventional orientation, together with an optically active layer to rotate the input polarization.
The foregoing embodiments of the invention described above involve rotation of the input polarization and LC structure into an orientation that is aligned with the horizontal/vertical edges of the unrotated pixel electrodes. An additional class of embodiments is possible wherein the edges of pixel electrodes are rotated into alignment with an LC structure that is kept in the conventional orientation.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.